Allah

One commonality among local Jews and Christians was that both groups spoke Arabic. As such, whenever Jews and Christians referred to the One God that they worshiped, the term they would have used in Arabic would have been Allah, derived from Al-ilah. Al is the definite article "the," and ilah is the Arabic word for "deity." In other words, Allah simply was the definitive noun meaning "the God." Today as in the past, Arabs (whether Muslim or Christian) use the term "Allah" to refer to the One God. Contrary to some assertions, Allah is not somehow the "Islamic" God as distinct from a Jewish or Christian deity. All Arabic speakers, both past and present, be they Christian, Jew or Muslim, use the term "Allah" to refer to God.

 -Memories of Muhammad: Why the Prophet Matters by Omid Safi, pg. 81